There is no doubt that Saddam used chemical weapons against Iranian soldiers and Kurdish villagers in the 1980s. UN inspectors sent to Iraq after the liberation of Kuwait destroyed ballistic missiles and thousands of chemical munitions and dismantled his nuclear weapons program but struggled to explain what happened to Iraq's biological programs.

Iraq played cat-and-mouse with inspectors until 1998, when UN arms experts withdrew just before a four-day American and British bombing campaign. The inspectors' brief return last year found Iraq had built missiles with ranges slightly longer than the 150-kilometre limit, but concluded Iraq had not rebuilt its nuclear program. They shed little light on chemical and biological programs.

What did the intelligence say?

The CIA and MI6 believed Saddam was rebuilding his weapons, but they were uncertain about how far he'd got. In a October 2002 report, the CIA said that Iraq had chemical and biological weapons as well as illegal missiles and "if left unchecked, it probably will have a nuclear weapon during this decade". Downing Street's dossier a month earlier said: "Iraq has a useable chemical and biological weapons capability, in breach of UN Security Council Resolution 687." It added that Iraq would be able to "deploy these weapons within 45 minutes of a decision to do so".

Did others disagree?

There was little dissent. Even opponents of the war, such as France and Germany, agreed there was a need to disarm Iraq but argued that UN inspectors should continue their work.

Scott Ritter, a former UN weapons inspector, was almost alone in arguing that it was doubtful that Saddam had retained any functional banned weapons.

What about the UN inspectors?

Successive reports by the UN described at length how tonnes of chemical weapons and chemical precursors, along with tens of thousands of chemical munitions, were "unaccounted for". They were particularly alarmed about possible stocks of VX gas, evidence suggesting Iraq was hiding more than 6000 chemical shells and the fate of a range of biological agents that Iraq could not prove had been destroyed. In the run-up to the war, chief UN weapons inspector Hans Blix complained about Iraq's failure to explain the many discrepancies in its claims. But he also criticised the poor quality of intelligence he was receiving from the US and Britain.

What has been found in Iraq?

The Iraq Survey Group, the 1400-member team headed by the former CIA official David Kay, has scoured the country for eight months but has yet to find any stocks of chemical or biological weapons. Years of sanctions and UN inspections appear to have contained Saddam's weapons programs, albeit at dreadful human cost.

Kay has said it is highly unlikely that any large stockpiles of weapons will be found. Although Mr Kay was adamant that Saddam had the "intention" to rearm once he was able to do so, and had tried to hide a wide range of illegal weapons-related activity, he probably destroyed all his remaining stocks of illegal weapons in the mid-1990s.

Why was the intelligence so wrong?

Mr Kay said there was "limited data" on Iraq and the US relied too heavily on satellite and signals intelligence rather than on human spies. There was also an acute shortage of Arabic-speaking spies. The US also relied heavily on intelligence provided by foreign agencies and Iraqi defectors.

So are the politicians free of blame?

Not entirely. While the intelligence agencies are likely to have overestimated the threat, the politicians are under pressure to explain why they claimed that Saddam posed such a large and imminent threat that he had to be deposed by force of arms.

Why did Saddam not show he had no WMD?

This is the central mystery: if Saddam had destroyed his weapons, why not try to save his regime by presenting the evidence? Kay offered two possible explanations: Saddam was bluffing and wanted his enemies to believe he still had the weapons, or he was fooled by corrupt officials who pretended to be building the weapons while stealing the funds.

Saddam clearly behaved like a man who had something to hide. The bizarre truth may be that what he was hiding was not weapons of mass destruction, but the fact that he had none.